Frahm, J.

Haenicke, W.

Merboldt, K.

English

Merboldt, K. D.

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The effects of intravoxel dephasing and incomplete slice refocusing on susceptibility contrast in gradient-echo MRI

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JOURNAL OF MAGNETIC RESONANCE, Series B 109, 234–237 (1995)The Effects of Intravoxel Dephasing and Incomplete Slice Refocusingon Susceptibility Contrast in Gradient-Echo MRIJENS FRAHM,* KLAUS-DIETMAR MERBOLDT, AND WOLFGANG HÄNICKEBiomedizinische NMR Forschungs GmbH am Max-Planck-Institut für biophysikalische Chemie, Am Faßberg 11, D-37077 Göttingen, GermanyReceived August 7, 1995In recent years, the sensitivity of gradient-echo MRI se- phase distribution which—at a given echo time—emergesquences to magnetic field inhomogeneities and tissue suscep- from a corresponding field or frequency distribution. Figurestibility differences has found an increasing number of appli- 1 and 2 show gradient-echo images of a water phantomcations ranging from an assessment of trabecular bone struc- and the brain of a healthy human subject ( informed writtenture to monitoring intravascular contrast agents or consent) , respectively, that have been acquired in the ab-visualizing brain function. In particular, gradient-echo MRI sence and presence of macroscopic magnetic field gradients.sequences sensitized to alterations of cerebral blood oxygen- The images were obtained at 2.0 T (Siemens Magnetomation (CBO) have resulted in exciting new tools for high- SP4000) with use of the standard imaging head coil, togetherresolution mapping of human brain activation. The underly- with spin-density-weighted RF-spoiled FLASH sequencesing ‘‘functional contrasts’’ are due to regional and transient with T*2 sensitivity under conditions that are identical tochanges in the concentration of deoxyhemoglobin. Although those commonly employed for CBO-sensitive functionalmany studies have addressed the physiologic origin of MRI brain mapping in this laboratory (3, 4) .susceptibility contrasts and developed models of underlying In the absence of a gradient, Figs. 1a and 2a exhibit amagnetic field (or frequency) distributions, little attention certain degree of inhomogeneity-induced signal loss thathas been paid to the way in which such effects become originates from intravoxel dephasing at the relatively longmodulated by the imaging process itself. As a consequence, gradient-echo time. The images serve as references for thethe influence of the slice-selection gradient on pertinent gra- effects of a gradient along the frequency-encoding axisdient-echo images has been largely overlooked. (Figs. 1b and 2b), the slice-selection axis (Figs. 1c and 2c),In a previous study, the signal loss of a gradient-echo and the phase-encoding axis (Figs. 1d–1f and 2d–2f) . Inimage that stems from an additional macroscopic magnetic line with previous investigations (1, 2) , a further reductionfield inhomogeneity was entirely attributed to a miscalibra- in MRI signal intensity was only seen for the miscalibratedtion of the slice-selection gradient (1) . More recently, we gradient in the slice direction. A gradient in the frequency-have identified the strength of the slice-selection gradient as encoding axis results in stretching or shrinking of the objecta factor determining the quality of functional difference along that dimension (depending on relative signs) , whilemaps (2) . Here, we further strengthen the analysis of suscep- its application along the perpendicular phase-encoding direc-tibility contrasts by discussing the influence of all three im- tion rotates the effective read gradient to an oblique axis.aging gradients. We demonstrate that, in general, gradient- By comparing respective image raw data sets in Fig. 3, it isecho MRI signal changes reflect contributions not only from seen that inhomogeneity components in the frequency- and‘‘intravoxel dephasing,’’ but also from incomplete refocus-phase-encoding direction shift the gradient echo along corre-ing of the slice-selection gradient. Moreover, while intra-sponding directions in k space. Fourier transformation resultsvoxel dephasing originates from an increase in broadness ofin altered signal phases, but does not lead to signal loss inthe underlying field or frequency distribution, incompletemagnitude images. Of course, if very strong inhomogeneitiesslice refocusing results from an offset in its mean frequency.push a major portion of the echo outside the acquisitionIn a first step, we experimentally confirm that a homoge-window, the loss of low spatial frequencies results in a com-neous shift of the entire frequency distribution results inplete lack of gross structures and a concomitant edge en-signal reduction due to incomplete slice refocusing as it ishancement (e.g., see Figs. 1e, 1f and 2e, 2f) . This phenome-less known than signal loss due to vectorial summation overnon has already been noted for strongly asymmetric echoesan ensemble of nuclear spin moments with a broadenedalong the frequency- (5) or phase-encoding direction (6, 7) .In a second step, we summarize our experimental under-standing of inhomogeneity-related signal losses in gradient-* To whom correspondence should be addressed.2341064-1866/95 $12.00Copyright q 1995 by Academic Press, Inc.All rights of reproduction in any form reserved.m4235$7293 09-28-95 06:59:15 maga AP: Mag Res235COMMUNICATIONSFIG. 1. Susceptibility-sensitized, cross-sectional, gradient-echo images of a water-filled beaker (2.0 T, RF-spoiled FLASH, TR/TE Å 62.5/30 ms,107 flip angle, measured data matrix 96 1 256, field-of-view 150 1 200 mm, slice thickness 4 mm, measuring time 6.0 s) . (a) Reference. (b) Applicationof a 0.1 mT m01 gradient (corresponding to a 0.75 A offset of the linear shim current) along the frequency-encoding direction (vertical) . (c) Applicationof a 0.1 mT m01 gradient along the slice-selection direction. (d–f) Application of a 0.1, 0.28, and 0.32 mT m01 gradient along the phase-encodingdirection (horizontal) , respectively.echo MRI in an integrated perspective. As indicated in Table preclude most spins from refocusing as it causes a net phaseoffset that further increases with echo time.1, a magnetic field inhomogeneity may alter the field orfrequency distribution of nuclear spin moments in an image Intravascular susceptibility changes in human brain thatare associated with activation-related changes in deoxyhem-voxel in two ways, i.e., by changing its broadness and byshifting its mean value. Although details depend on the ac- oglobin concentration cause both a change in broadness (8–12) and mean (13, 14) of the field or frequency distributiontual mechanism, such alterations generally occur in all threespatial dimensions and therefore independently affect the of affected spins. Although the relative importance of respec-tive MRI signal changes depends on a large number of un-imaging gradients. As far as frequency- and phase-encodinggradients are concerned, intravoxel signal changes are en- known variables, the slice-selection process is expected tobe particularly sensitive as it benefits from both intravoxeltirely due to a change in broadness of the frequency distribu-tion, while a change in mean frequency shifts the echo posi- dephasing and incomplete slice refocusing and thus maybe exploited to amplify CBO-related signal alterations. Fortion and leads to geometric distortions during image recon-struction. Conversely, however, both types of distribution example, these considerations have consequences for opti-mizing volume coverage in CBO-sensitive functional brainchanges alter the MRI signal intensity when present in theslice-selection direction. While intravoxel dephasing is often mapping as multislice approaches are likely to be preferableover 3D imaging. This is a consequence of the fact that ineven stronger than in the frequency- and phase-encodingdirection as section thickness usually surpasses in-plane 3D gradient-echo MRI both the 2D and 3D phase-encodinggradient may at least partially compensate for magnetic fieldvoxel dimensions, a shift of the entire distribution may easilym4235$7293 09-28-95 06:59:15 maga AP: Mag Res236 COMMUNICATIONSFIG. 2. Susceptibility-sensitized, cross-sectional, gradient-echo images of the brain of a normal volunteer (parameters as described in the legend toFig. 1) . (a) Reference. (b) Application of a 0.1 mT m01 gradient along the frequency-encoding direction (vertical) . (c) Application of a 0.1 mT m01gradient along the slice-selection direction. (d–f) Application of a 0.1, 0.25, and 0.28 mT m01 gradient along the phase-encoding direction (horizontal) ,respectively.FIG. 3. Raw data (magnitude representation) of the gradient-echo images shown in Fig. 1. (a) Reference (data matrix 96 1 256, asymmetric echo).(b) Application of a 0.1 mT m01 gradient along the frequency-encoding direction (horizontal shift in k space) . (c) Application of a 0.1 mT m01 gradientalong the slice-selection direction (no shift but reduced signal-to-noise) . (d–f) Application of a 0.1, 0.28, and 0.32 mT m01 gradient along the phase-encoding direction (vertical shifts in k space) , respectively.m4235$7293 09-28-95 06:59:15 maga AP: Mag Res237COMMUNICATIONSTABLE 1 instrumental, structural, or functional origin of the field inho-The Consequences of Magnetic Field Inhomogeneities in a mogeneity.Gradient-Echo ImageREFERENCESChange in field or frequency distribution1. J. Frahm, K. D. Merboldt, and W. Hänicke, Magn. Reson. Med. 6,MRI gradient Broadness Mean 474 (1988).2. J. Frahm, K. D. Merboldt, and W. Hänicke, J. Magn. Reson. B 103,Frequency encoding Intravoxel dephasing Echo shift 91 (1994).(r signal loss) (r image distortion)3. J. Frahm, K. D. Merboldt, and W. Hänicke, Magn. Reson. Med. 29,Phase encoding Intravoxel dephasing Echo shift139 (1993).(r signal loss) (r image distortion)4. J. Frahm, K. D. Merboldt, W. Hänicke, A. Kleinschmidt, and H.Slice selection Intravoxel dephasing Incomplete refocusingBoecker, NMR Biomed. 7, 45 (1994).(r signal loss) (r signal loss)5. G. C. Hurst, J. Hua, O. P. Simonetti, and J. L. Duerk, Magn. Reson.Med. 27, 247 (1992).Note. Changes in broadness or mean of the underlying field or frequencydistribution cause either signal loss or geometric distortions, depending on 6. J. H. den Boef, C. M. van Uijen, and C. D. Holzscherer, Phys. Med.the functionality of the MRI gradients. Biol. 29, 857 (1984).7. P. Margosian and F. Schmitt, Proc. SPIE: Med. Image Process.593, 6 (1985).8. C. R. Fisel, J. L. Ackerman, R. B. Buxton, L. Garrido, J. W. Belliveau,inhomogeneities in the manner demonstrated in Figs. 1d, 2d, B. R. Rosen, and T. J. Brady, Magn. Reson. Med. 17, 336 (1991).and 3d. The more extended the acquisition window along 9. S. Ogawa, R. S. Menon, D. W. Tank, S. G. Kim, H. Merkle, J. M.Ellerman, and K. Ugurbil, Biophys. 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The effects of intravoxel dephasing and incomplete slice refocusing on susceptibility contrast in gradient-echo MRI

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